In the field of lubricating oils, additives such as pour point depressants have conventionally been added to lubricating base oils including highly refined mineral oils, to improve the properties such as the low-temperature viscosity characteristic of the lubricating oils (see Patent documents 1-3, for example). Known processes for production of high-viscosity-index base oils include processes in which stock oils containing natural or synthetic normal paraffins are subjected to lubricating base oil refining by hydrocracking/hydroisomerization (see Patent documents 4-6, for example).
Evaluation standards of the low-temperature viscosity characteristic of lubricating base oils and lubricating oils are generally the pour point, clouding point and freezing point. Methods are also known for evaluating the low-temperature viscosity characteristic based on the lubricating base oils, according to their normal paraffin or isoparaffin contents.    [Patent document 1] Japanese Unexamined Patent Publication HEI No. 4-36391    [Patent document 2] Japanese Unexamined Patent Publication HEI No. 4-68082    [Patent document 3] Japanese Unexamined Patent Publication HEI No. 4-120193    [Patent document 4] Japanese Unexamined Patent Publication No. 2005-154760    [Patent document 5] Japanese Patent Public Inspection No. 2006-502298    [Patent document 6] Japanese Patent Public Inspection No. 2002-503754